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No brake pressure  Rating:  Rating
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 Posted: 04-25-2007 01:30 am
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Bob 13902
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Winter is over, jumped into my Jensen Healey and brake pedal went to the floor. No brake fluid anywhere. Everything was fine when I put it away in the fall. Late last summer, I had replaced brake hoses due to front caliper not wanting to release (fixed that problem) and changed fluid and had good, strong brakes when I put it away. So I assumed the master cylinder was culprit - noted some slight leakage into booster area when I removed it - so installed a new Lucas TR-6 unit. Attempted to bleed and brakes acting same...right to the floor. Only fluid coming out is from rear brakes...and they sounded as if they were working although I couldn't feel it in the brake pedal. I can hear pumping air sound near front. I've never had to prime a master cylinder before, so I haven't yet tried that yet. When pumping the pedal and with the cap removed, watching the rear hole in the mastercylinder reservoir, I can see what looks like a tiny bubble (aspiration) each pump. Any ideas or secrets anyone knows that can help?

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 Posted: 04-25-2007 06:41 pm
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Greg Fletcher
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Sometimes it takes a while. I'd start bleeding the front driver side first. You may need to keep at a while to get all the air out. Tighten that up and do the passenger front and then the rear. Then do the front pair together. That should remove any extra air trapped inside. I always use one-way bleed valves on the end of a plastic tube to insure air can't go back in.

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 Posted: 04-25-2007 07:18 pm
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Jensen Healey
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Tip-o-the-day: I've had air leak at the threads when using the purge valves. A bit of heavy grease on the threads kept the air from being sucked in on the pedal up stroke.

I did have trouble bleeding the system after installing the TR6 MC without bench bleeding first. Persistence, several quarts of brake fluid and Hawk street pads pay off.

Kurt

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 Posted: 04-28-2007 01:13 pm
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Bob 13902
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Thanks - I pulled the master cylinder back out and bench bled after getting tired of exercising my foot. Putting a slight back pressure on the outlets by holding the cleanliness plugs partially in during cycling the piston made things go quick...a tip a friend from work suggested. The one pain with the TR6 backfit is the reservoir angle coupled with the shallow depth which causes the rear drain hole to uncover quickly when bleeding (have to keep the level high with frequent filling during bleeding...I uncovered it once because it is so hard to see the fluid depth with new fluid, having to start over). I also think the tiny opening to fill the forward chamber seems to be an odd feature.

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 Posted: 04-28-2007 03:20 pm
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Jensen Healey
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I noticed the same things when I installed the TR6 MC. Sure is great to have a bolt-on solution though.

Kurt

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 Posted: 05-07-2007 10:17 pm
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Bob 13902
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Got that new TR-6 master cylinder and everything working fine for a few days. Then, for no apparent reason, after a 5 mile drive, I noted the brake pads didn't seem to be fully releasing (car slowing down quicker than normal). Nothing was hot, but it was clear the front brakes (both) were on. A quick crack of the bleeder on one of the fronts and they were both free. Went back and bled everything and all seemed OK...even after another 5 mile drive. The next 5 mile drive (later in the day), all of a sudden I sensed brakes dragging. While looking for a place to stop (a mile or so I continued), they started going on more and more by themselves (I'm thinking brake fluid is heating up and putting more pressure on pad). I stop and things are smoking hot. I crack a bleeder, and pop, they release and I get home (another mile) fine. Let's see, I changed both front brake hoses a few hundered miles ago, so doubt that is the problem. Since both calipers were on and the master cylinder is the only new change, I suspect it has something to do with what is going on. Anybody got an idea for me to attack? Could the fluid being too high lead to this...I can think of no reason, and no differential pressure when unscrewed cap? With the reservoir being tilted, I filled it such that the fluid levels are at the top of the reservoir divider (so back one doesn't uncover). Maybe the booster rod needs adjust...thinking about it, I think I felt a slight resistance when installing the master cylinder, such that the piston may have been pushed in slightly when I bolted it up. Is that rod adjustable?

Here's an update...it was the servo rod adjustment. I had to screw it in almost the whole way (another 1/4" from setting with the original master cylinder).

Last edited on 05-12-2007 10:13 pm by Bob 13902

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 Posted: 05-09-2007 08:15 pm
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Jon Plowe
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I once had a very similar problem on another 70's car and if I remember correctly (increasingly unlikely!) that was caused by a fault in the servo unit.

Jon 

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